Gradient function material seal cap for discharge lamp bulb

ABSTRACT

A disclosed gradient function material seal cap for discharge lamp bulb is produced by molding and thereafter firing a slurry which contains a plurality of groups of particles having different specific gravities. The plurality of groups of particles include at least a first group of particles and a second group of particles. The first group of particles comprises a group of non-metal particles having a specific gravity ranging from about 3 to 7 and a maximum particle diameter equal to or smaller than a deflocculation limit, said nonmetal particles being made of one or more materials selected from the group consisting of an oxide, a carbide, a nitride, and an oxynitride. The second group of particles comprises a group of metal particles having a specific gravity which is about 1.5 times the specific gravity of said first group of particles, and particle diameters distributed across the deflocculation limit. The gradient function material seal cap is manufactured by preparing a slurry containing a plurality of groups of particles and/or a plurality of slurries, and supplying the slurry or slurries into a porous mold to form a deposited region in the porous mold initially primarily influenced by way of attraction of the porous mold and subsequently primarily influenced by a deflocculating effect of the particles in the slurry or slurries or under the influence of the gravity.

This is a divisional of prior pending U.S. application Ser. No.08/564,777, filed Nov. 29, 1995, which is a divisional of priorapplication Ser. No. 08/246,134, filed May 19, 1994 (now U.S. Pat. No.5,653,924), which is a continuation of PCT application PCT/JP93/01367,filed Sep. 24, 1993.

TECHNICAL FIELD

The present invention relates to a gradient function material whosecharacteristics continuously vary transversely thereacross, and a methodof manufacturing such a gradient function material.

BACKGROUND ART

It has heretofore been known to use a gradient function material as aheat resistant material in locations where the difference between innerand outer temperatures is very large, e.g., as a surface layer materialfor a space shuttle.

Conventional gradient function materials have been produced by eithermaking, laminating, pressing, and thereafter firing a number of greensheets which have slightly different compositions, or an evaporationprocess such as CVD or the like.

However, a gradient function material which is produced by laminatinggreen sheets does not have a completely continuous range ofcompositions, but has compositions varying stepwise transverselythereacross. Therefore, such a gradient function material fails to fullyperform the desired function thereof.

A gradient function material which is manufactured by an evaporationprocess such as CVD or the like can have a completely continuous rangeof compositions. However, since the thickness of a film that can bedeposited in one evaporation process is very small, it is very difficultfrom the standpoint of cost and technology to obtain a gradient functionmaterial having a required thickness.

There have been proposed methods of achieving a completely continuousrange of compositions and maintaining a desired material thickness asdisclosed in Japanese laid-open patent publications Nos. 3-165832 and3-274105.

According to the method disclosed in Japanese laid-open patentpublication No. 3-165832, a first slurry containing Ti particles or thelike and a second slurry containing SiC particles or the like areprepared and supplied to a filtering tank which has a filter, whilecontinuously varying the mixing ratio of the first and second slurries.When the slurries are drawn by a vacuum pump, a cake having a gradientcomposition is formed on the filter. The cake is then formed to shapewhile at the same time it is being dehydrated, after which the shapedcake is fired.

According to the method shown in Japanese laid-open patent publicationNo. 3-274105, a slurry is prepared which contains a plurality of typesof particles having different particle size distributions. The slurry isput in a container made of film (corresponding to the filter disclosedin Japanese laid-open patent publication No. 3-165832), and a cakehaving a gradient composition is formed in the container by centrifugalseparation or sedimentation. The cake is then formed to shape while atthe same time it is being dehydrated, after which the shaped cake isfired.

However, the apparatus which are required are complex and expensivebecause the vacuum pump and a rotating device for generating thecentrifugal force are necessary.

The gradient layer that is formed is of an increased total thickness ofseveral millimeters as necessitated by the handling of the cake.Therefore, the gradient layer has a large heat capacity and is of astructure vulnerable to a heat shock.

FIG. 24 is an enlarged view showing the manner in which particles areattracted when a filter or a container of film is used. Particles 202are concentrated on holes 201 in a filter 200, creating gaps in regionsother than the holes 201. Therefore, a cake that is produced has a watercontent of 30% or more. Since such a cake cannot directly be fired, ithas heretofore been customary to dehydrate the cake, making themanufacturing process complicated.

When a cake is dehydrated and dried, the product tends to crack andshrinks to a very large degree due to dehydration and drying. Therefore,it is necessary to cut out the product after the cake is dehydrated anddried.

FIG. 25 is a cross-sectional view of a gradient layer 204 which isformed according to a conventional method. Inasmuch as particles areattracted under suction forces applied in a constant direction in theconventional method, the laminae of the gradient layer 204 are arrangedas horizontal stripes, and are liable to peel off because they aresuper-posed too orderly in the transverse direction.

Furthermore, gradient function materials manufactured according to theconventional methods have shapes that are limited to simple shapes.Specifically, according to the conventional methods, since water has tobe removed from a cake in a pressing step, the gradient layer would bedeformed out of an orderly configuration when dehydrated if it were of acomplex shape.

Consequently, the prior methods proposed in Japanese laid-open patentpublications Nos. 3-165832 and 3-274105 suffer the following drawbacks:

a) As the vacuum pump and the rotating device for generating thecentrifugal force are necessary, the apparatus is complex and expensive.

b) The gradient layer which is formed is of a large thickness of severalmillimeters as necessitated by the handling of the cake. Therefore, thegradient layer has a large heat capacity and is of a structurevulnerable to a heat shock.

c) If a filter or a film is used, the water content of a cake is large(30% or more). Because such a cake cannot directly be fired, it isindispensable to dehydrate the cake, making the manufacturing processcomplex.

d) When a cake is dehydrated and dried, the product tends to crack andshrinks to a very large degree due to dehydration and drying. Therefore,it is necessary to cut out the product after the cake is dehydrated anddried.

e) Inasmuch as particles are attracted under suction forces applied in aconstant direction, the laminae of the gradient layer are arranged ashorizontal stripes, and are liable to peel off because they aresuperposed too orderly in the transverse direction.

f) Gradient function materials manufactured according to theconventional methods have shapes that are limited to simple shapes.Specifically, according to the conventional methods, since water has tobe removed from a cake in a pressing step, the gradient layer would bedeformed out of an orderly configuration when dehydrated if it were of acomplex shape.

g) The composition varies such that a certain component either increasesor decreases in the transverse direction. Therefore, it has not beenconventionally possible to obtain any gradient function materials whichhave a composition peak intermediate in their transverse direction.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the foregoing problemsattendant conventional gradient function materials and the conventionalmethods and apparatus for forming same. It is therefore an object of thepresent invention to provide a gradient function material which iscomposed of particles having a composition continuously varying in atransverse direction as if in transversely super-posed corrugatedpatterns, and which is made from a single slurry containing a group ofparticles having diameters according to a deflocculating effect or amixed slurry containing a plurality of types of particles using a porousmold such as a plaster mold rather than a filter or a film, and a methodof manufacturing such a gradient function material.

According to the present invention, a gradient function material isproduced by molding and thereafter firing a slurry which contains aplurality of groups of particles having different specific gravities,characterized in that the plurality of groups of particles includes atleast a first group of particles and a second group of particles, thefirst group of particles comprises a group of nonmetal particles havinga specific gravity ranging from about 3 to 7 and a maximum particlediameter equal to or smaller than a deflocculation limit, the nonmetalparticles being made of one or more materials selected from the groupconsisting of an oxide, a carbide, a nitride, and an oxynitride, and thesecond group of particles comprises a group of metal particles having aspecific gravity which is about 1.5 times the specific gravity of thefirst group of particles, and particle diameters distributed across thedeflocculation limit.

The first group of particles may preferably comprise a group of nonmetalparticles having a specific gravity ranging from about 3 to 7 and amaximum particle diameter equal to or smaller than a deflocculationlimit of about 6.0 μm, the non-metal particles being made of one or morematerials selected from the group consisting of alumina, zirconia,magnesia, silica, silicon carbide, titanium carbide, silicon nitride,and AlON, and the second group of particles may comprise a group ofmetal particles of a high melting point which have a specific gravitywhich is about 1.5 times the specific gravity of the first group ofparticles, and particle diameters distributed across the deflocculationlimit of about 6.0 μm, the metal particles being made of an alloycontaining at least one material selected from the group consisting ofnickel, tungsten, molybdenum, tantalum, and chromium.

The gradient function material according to the invention may bemanufactured by the steps of preparing a first slurry containing atleast a group of small-specific-gravity particles having a smallspecific gravity and a second slurry containing at least a group oflarge-specific-gravity particles having a large specific gravity,supplying a quantity of one of the first slurry and the second slurrysingly into a porous mold to form a deposited region in the porous mold,and mixing quantities of the first slurry and the second slurry into amixed slurry and supplying the mixed slurry into the porous mold toallow the particles to be deposited successively from those particles inthe mixed slurry which are more susceptible to gravity, onto thedeposited region.

The gradient function material according to the invention may also bemanufactured by the steps of preparing a first slurry containing atleast a group of small-specific-gravity particles having a smallspecific gravity and a second slurry containing at least a group oflarge-specific-gravity particles having a large specific gravity, mixingthe first slurry and the second slurry into a mixed slurry and supplyingthe mixed slurry into a porous mold to allow the particles to bedeposited successively from those particles which are more susceptibleto gravity.

If the porous mold comprises a plaster mold, then a gradient layerhaving a water content of about 5%, rather than a cake having a watercontent of 30% or more which has heretofore been produced using afilter, is deposited directly in the mold.

The gradient function material may be arranged to have such acontinuously varying composition that there exists alarge-specific-gravity region which is occupied mostly by a group oflarge-specific-gravity particles having a greatest specific gravity,among the groups of particles, between upper and lower ends thereof inthe transverse direction, and a proportion of a group ofsmall-specific-gravity particles having a small specific gravityprogressively increases transversely from the large-specific-gravityregion toward both ends in the transverse direction.

The gradient function material according to the invention may further bemanufactured by the steps of preparing a single slurry containing atleast a group of small-specific-gravity particles having a smallspecific gravity and a mixed slurry containing at least the group ofsmall-specific-gravity particles and a group of large-specific-gravityparticles having a large specific gravity, supplying the single slurryinto a porous mold to form a deposited region made of the group ofsmall-specific-gravity particles in the porous mold, and supplying themixed slurry into the porous mold to allow the particles to be depositedsuccessively from those particles in the mixed slurry which are moresusceptible to gravity, onto the deposited region made of the group ofsmall-specific-gravity particles, thereby forming a gradient functionmaterial forming body. In addition, a step of taking the gradientfunction material forming body out of the porous mold and removing thedeposited region made of the group of small-specific-gravity particlesfrom the gradient function material forming body may be included.

If the porous mold comprises a plaster mold, then when a mixed slurrycontaining a group of particles having a small specific gravity and agroup of particles having a large specific gravity is supplied into theplaster mold, a deposited region is initially formed in contact with asurface of the mold as dominated by being attracted by the plaster mold,but not by a specific gravity and a deflocculated condition, thedeposited region having a composition substantially equal to thecomposition of the slurry. After the deposited region is formed thereon,another deposited region is formed as dominated by the specific gravityand the deflocculated condition. Accordingly, the gradient functionmaterial can be manufactured which has a composition peak in anintermediate region thereof. Other objects, advantages and salientfeatures of the present invention will become apparent from thefollowing detailed description which, when taken in conjunction with theannexed drawings, discloses preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a flowchart of a process of preparing a nickel slurry usedin the manufacture of a gradient function material in a method ofmanufacturing a gradient function material according to a firstembodiment of the present invention;

FIG. 1(b) is a flowchart of a process of preparing an alumina slurryused in the manufacture of a gradient function material in the abovemethod of manufacturing a gradient function material;

FIG. 2 is a schematic view of a manufacturing apparatus for experimentaluse in the method of manufacturing a gradient function materialaccording to the first embodiment;

FIG. 3 is an enlarged view showing the deposited condition of adeposited layer on a plaster mold, which is produced by themanufacturing apparatus shown in FIG. 2;

FIG. 4 is a graph showing a deflocculating effect and a particle sizedistribution of each of a nickel slurry and an alumina slurry for use inthe method of manufacturing a gradient function material according tothe first embodiment;

FIG. 5 is a cross-sectional view of a gradient layer in a gradientfunction material that is produced by the method of manufacturing agradient function material according to the first embodiment;

FIG. 6(a) is a graph showing the deposited layer growth rates on aplaster mold of nickel particles when they are in a deflocculatedcondition and not in a deflocculated condition;

FIG. 6(b) is a graph showing the deposited layer growth rate on aplaster mold of alumina particles when they are in a deflocculatedcondition;

FIG. 6(c) is a graph showing the deposited layer growth rate on aplaster mold of a mixture of nickel and alumina particles;

FIG. 7 is a cross-sectional view of an actual manufacturing apparatusfor use in the method of manufacturing a gradient function materialaccording to the first embodiment;

FIG. 8 is a view as viewed in the direction indicated by the arrow 8 inFIG. 7;

FIG. 9 is a cross-sectional view of a gradient function materialmanufactured by the manufacturing apparatus shown in FIG. 7;

FIG. 10 is a comparison table showing characteristics of a gradientfunction material manufactured by the method of manufacturing a gradientfunction material according to the first embodiment and conventionalgradient function materials after they are fired;

FIG. 11 is a comparison table showing characteristics of the method ofmanufacturing a gradient function material according to the firstembodiment and conventional methods;

FIG. 12(a) is a flowchart of a basic manufacturing process in a methodof manufacturing a gradient function material according to a secondembodiment of the present invention;

FIG. 12(b) is a flowchart of a manufacturing process which employs asintering assistant different from a sintering assistant used in themanufacturing process shown in FIG. 12(a);

FIG. 13(a) is a graph showing a particle size distribution of Al₂ O₃used in the manufacturing process shown in FIG. 12(a);

FIG. 13(b) is a graph showing a particle size distribution of W used inthe manufacturing process shown in FIG. 12(a);

FIG. 13(c) is a graph showing a particle size distribution of NiCr usedin the manufacturing process shown in FIG. 12(a);

FIGS. 14(a) through 14(e) are cross-sectional views showing a process ofmanufacturing a sealing cap for a metal vapor discharge lamp as agradient function material according to the manufacturing process shownin FIG. 12(a);

FIG. 15 is an enlarged cross-sectional view of a gradient functionmaterial manufactured by the method of manufacturing a gradient functionmaterial according to the second embodiment;

FIG. 16(a) is a cross-sectional view of a bulb for a metal vapordischarge lamp using the sealing cap manufactured according to themanufacturing process shown in FIG. 12(a);

FIG. 16(b) is a fragmentary cross-sectional view of a sealing cap whichis of a type different from the sealing cap shown in FIG. 16(a);

FIGS. 17(a) and 17(b) are views showing a method of manufacturing agradient function material according to a first modification of themethod of manufacturing a gradient function material according to thesecond embodiment;

FIG. 18 is a graph showing a composition in the transverse direction ofa gradient function material forming body shown in FIG. 14(b);

FIG. 19 is a graph showing a composition in the transverse direction ofa gradient function material forming body shown in FIG. 17(b);

FIGS. 20(a) and 20(b) are views showing a method of manufacturing agradient function material according to a second modification of themethod of manufacturing a gradient function material according to thesecond embodiment;

FIG. 21 is a graph showing the relationship between the depositedthickness and the electric resistance of the gradient function materialforming body shown in FIG. 14(b);

FIG. 22 is a graph showing the relationship between the depositedthickness and the Vickers hardness of the gradient function materialforming body shown in FIG. 14(b);

FIG. 23 is a graph showing the relationship between the depositedthickness and the fracture toughness value of the gradient functionmaterial forming body shown in FIG. 14(b);

FIG. 24 is an enlarged view showing the manner in which a group ofparticles in a slurry are attracted to a filter used in a conventionalmethod of manufacturing a gradient function material; and

FIG. 25 is a cross-sectional view of a gradient layer in a gradientfunction material produced by a conventional method of manufacturing agradient function material.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of gradient function materials according to thepresent invention will hereinafter be described below with reference tothe drawings.

FIGS. 1(a) and 1(b) are flowcharts of processes of preparing a nickelslurry and an alumina slurry, respectively, used in the manufacture of agradient function material in a method of manufacturing a gradientfunction material according to a first embodiment of the presentinvention. The particle diameters, specific gravities, and other data ofnickel and alumina powders as materials are shown in the table 1 below.

To prepare a nickel slurry, a mixture of pure water, a deflocculant, abinder, and an antifoamer which are weighed as shown in FIG. 1(a) and anickel powder which is weighed as shown in FIG. 1(a) are mixed in amonopot, after which an antifoamer is added to the mixture. The mixtureis then deaerated in vacuum and sieved to remove floating substances andcontaminants, thus producing a nickel slurry. Similarly, an aluminaslurry is produced by weighing, mixing, and processing the materials asshown in FIG. 1(b).

                  TABLE 1                                                         ______________________________________                                                       Nickel powder                                                                          Alumina powder                                        ______________________________________                                        Average particle diameter (μm)                                                              1.2        0.5                                               Particle size distribution (μm)                                                             0.3 ˜ 9.0                                                                          0.1 ˜ 2.0                                   Shape (-)        Nearly sphere                                                                            Nearly sphere                                     Specific gravity (-)                                                                           9.845      3.986                                             Purity (wt %)    99.50      99.99                                             Resistivity (Ω·cm/0° C.)                                                 6.8 × 10.sup.-6                                                                    1.0 × 10.sup.14                             Coefficient of thermal                                                                         12.8       7.0                                               expansion × 10.sup.-6 /°C.                                       ______________________________________                                    

The deflocculant comprises polycarboxylic acid (A6114 manufactured byToagosei Chemical Industry Co., Ltd.). The binder comprises CMC(carboxymethyl cellulose, which is sold as SMR-10M manufactured by TheShin-Etsu Chemical Co., Ltd.). The antifoamer comprises polyglycol(CE-457 manufactured by Nippon Oils & Fats Co., Ltd).

FIG. 2 is a schematic view of a manufacturing apparatus which is usedprimarily for experimentation and can be used to manufacture a gradientfunction material. A process for manufacturing a gradient functionmaterial using nickel and alumina slurries which have been preparedaccording to the above processes will be described below.

A glass tube 20 coated with wax on its inner surface and having a heightof 30 mm and a diameter of 40 mm is set on a plaster mold 10 (such as amold which meets the requirements under JIS R9111) held at rest whichhas a height of 60 mm and a diameter of 100 mm. First, 3 ml. of nickelslurry S1 is poured into the glass tube 20. About 6 minutes later, adeposited layer having a thickness of 0.5 mm is formed on the plastermold 10.

FIG. 3 is an enlarged view showing the deposited condition of thedeposited layer. A comparison with the deposited condition shown in FIG.24 indicates that nickel particles 40 are uniformly deposited on thesurface of the plaster mold 10 because the area of pores per unitsurface area of the plaster mold 10 is far greater than the filter 200used in the conventional method.

Then, a mixed slurry S2 of 1.5 ml. of nickel slurry and 4 ml. of aluminaslurry is poured into the glass tube 20.

FIG. 4 is a graph showing a deflocculating effect and a particle sizedistribution of each of the nickel slurry and the alumina slurry. Asshown in the Table 1 above, the nickel slurry has a particle sizedistribution ranging from 0.3 μm to 9.0 μm, and the alumina slurry has aparticle size distribution ranging from 0.1 μm to 2.0 μm. The limitparticle diameter within which the deflocculating effect is exhibited isabout 6.0 μm.

As a result of the mixed slurry S2 which has been poured into the glasstube 20, the nickel particles which have a particle diameter of 6.0 μmor greater are deposited, and the nickel and alumina particles whichhave a particle diameter of 6.0 μm or smaller are suspended in theslurry while being repelled by each other.

However, the particles in the deflocculated condition are graduallydeposited due to gravity. Because the specific gravity of nickel islarger than (more than twice) the specific gravity of alumina, morenickel particles are deposited on the deposited layer of nickelparticles which has first been formed on the plaster mold 10 thanalumina particles. In this manner, a gradient layer 30 is formed asshown in the cross-sectional view of FIG. 5.

FIGS. 6(a) through 6(c) show the relationship between time and depositedthicknesses for the deposited layer growth rates of various particlegroups on the plaster mold 10. FIG. 6(a) is a graph showing thedeposited layer growth rates of nickel particles. In FIG. 6(a), thecurve a indicates the deposited layer growth rate of nickel particleswhich are not in a deflocculated condition (with no deflocculant), andthe curve β indicates the deposited layer growth rate of nickelparticles which are in a deflocculated condition (with a deflocculant).In FIG. 6(b), the curve γ represents the deposited layer growth rate ofalumina particles which are in a deflocculated condition (with adeflocculant). In FIG. 6(c), curve β+γ represents the actual depositedlayer growth rate in the manufacturing method according to the firstembodiment. In the first embodiment, the formation of a desireddeposited layer is completed in 25˜30 minutes.

The composition of the nickel and alumina particles in the gradientlayer 30 continuously varies in a transverse direction thereof as if intransversely superposed corrugated patterns. Specifically, thecomposition of the nickel and alumina particles varies in a corrugatedfashion in a direction normal to the transverse direction andcontinuously varies or is continuously inclined in the transversedirection. The reason for this is that the order (governed by thespecific gravity) of the gradient layer is disturbed because theparticles which have a particle size distribution of 6 μm or less aredeflocculated and less liable to be governed by the specific gravity.

FIG. 7 is a cross-sectional view of a manufacturing apparatus for use inthe method of manufacturing a gradient function material according tothe first embodiment, and FIG. 8 is a view as viewed in the directionindicated by the arrow 8 in FIG. 7.

In the manufacturing apparatus, a plaster mold 60 which can be dividedinto halves is set on a pair of rotatable rollers 70 which extendhorizontally, and one end of a cylindrical cavity 60a defined in theplaster mold 60 and a hopper 50 are interconnected by a pipe 50a and arotary joint 80. The other end of the cylindrical cavity 60a is closedby a cap 60b.

In operation, a nickel slurry S1 is supplied from the hopper 50 into thecavity 60a in the plaster mold 60, which is then rotated at 10˜15 rpm todeposit the slurry on an inner circumferential surface of the cavity60a. Thereafter, a mixed slurry S2 of a nickel slurry and an aluminaslurry is supplied from the hopper 50 into the cavity 60a, and depositedin the cavity 60a. In this manner, a tubular body 90 is produced whichis nickel-rich in its outer region, an alumina-rich in its inner region,and has a gradient material or layer in its intermediate region. Across-sectional view of the tubular body 90 is shown in FIG. 9.

In this embodiment, a first slurry is made of alumina particles and asecond slurry is made of nickel particles. However, SiC, zirconia, orthe like may be used instead of alumina particles, and W, Cr, Ta, or thelike may be used instead of nickel particles. Stated otherwise, a firstslurry may be made of a group of particles having a small specificgravity, and a second slurry may be made of a group of particles havinga large specific gravity.

The first group of particles having a small specific gravity maycomprise one or a plurality of groups of nonmetal particles having amaximum particle diameter equal to or smaller than the deflocculationlimit (6 μm), such as of an oxide such as alumina, zirconia, magnesia,silica, or the like, a carbide such as silicon carbide, titaniumcarbide, or the like, a nitride such as silicon nitride, and anoxynitride such as AlON or the like. The second group of particleshaving a large specific gravity may comprise a group of metal particlesof an alloy of nickel, tungsten, molybdenum, tantalum, chromium, or thelike, the metal particles having a high melting point, a specificgravity which is 1.5 times that of the first group of particles orgreater, and particle diameters distributed across the deflocculationlimit (6 μm).

FIG. 10 is a comparison table showing characteristics of a gradientfunction material manufactured by the method of manufacturing a gradientfunction material according to the first embodiment and conventionalgradient function materials after they are fired. FIG. 11 is acomparison table showing characteristics of the method of manufacturinga gradient function material according to the first embodiment andconventional methods.

As can be seen from the comparison tables of FIGS. 10 and 11 andunderstood from the above discussion of Background Art, the gradientfunction material according to the first embodiment can achieve arequired thickness in a much larger and complete range than the gradientfunction materials according to the conventional methods. Specifically,the gradient layer produced by vacuum evaporation such as CVD has amaximum thickness of several hundreds μm, and the gradient layerproduced by centrifugal separation or solid-liquid separation using afilter has a minimum thickness of several mm, whereby there is a largegap or empty region in the thicknesses for the gradient layer which maybe achieved using conventional methods. According to the presentmanufacturing method, it is possible to manufacture a gradient layerhaving a thickness ranging from several hundreds μm to ten mm or more.

With the method of manufacturing a gradient function material accordingto the first embodiment, the composition of particles making up theproduced gradient function material continuously varies in a transversedirection thereof as if in transversely superposed corrugated patterns.Therefore, the gradient function material according to the presentinvention is less liable to peel off in thin layers and is moreresistant to thermal stresses than the conventional gradient functionmaterials which are composed of laminae arranged regularly assubstantially horizontal stripes.

In the manufacturing method according to the first embodiment,furthermore, since a gradient layer is formed by being deposited in aplaster mold, any conventional hydrating step is not necessary. Becauseno hydrating step is required, the manufacturing process is simplified,and the gradient layer can keep its shape because a pressing step isalso not required. In addition, the gradient function material is notlimited to a plate shape, but can be formed to any desired shape.

In the manufacturing method according to the first embodiment, becauseno vacuum pump and no rotating device for producing centrifugal forces,or no filter is needed, the apparatus may be of a simplified design,which gives an advantage as to the cost.

A method of manufacturing a gradient function material according to asecond embodiment of the present invention and a gradient functionmaterial manufactured according to such a method will be described belowwith reference to FIGS. 12(a) and 12(b) through 23.

FIG. 12(a) is a flowchart of a basic manufacturing process in a methodof manufacturing a gradient function material according to a secondembodiment of the present invention. In FIG. 12(a), a group of particlesof Al₂ O₃ is used as a group of particles having a small specificgravity, a group of particles of W is used as a group of particleshaving a large specific gravity, and a sintering assistant of NiCr(80/20) is used.

As shown in FIG. 13(a), particles of Al₂ O₃ having a particle diameterof 0.5 μm or less are selected. Since the limit particle diameter withinwhich a deflocculating effect is exhibited with respect to Al₂ O₃ isabout 6.0 μm, all particles of Al₂ O₃ having a particle diameter of 0.5μm or less are deflocculated.

The specific gravity of Al₂ O₃ is 3.99 (20° C.). Particles having asmall specific gravity (small-specific-gravity particles) which may beemployed instead of Al₂ O₃ are preferably one or a plurality of groupsof nonmetal particles having a specific gravity ranging from 3 to 7 anda maximum particle diameter equal to or smaller than the deflocculationlimit, such as of an oxide such as zirconia, magnesia, silica, or thelike, a carbide such as silicon carbide, titanium carbide, or the like,a nitride such as silicon nitride, and an oxynitride such as AlON or thelike.

Particles of W (tungsten) having a particle diameter ranging from 0.5 μmto 2.3 μm are selected as shown in FIG. 13(b). Since the limit particlediameter within which a deflocculating effect is exhibited with respectto W is about 1.0 μm, some particles of W are deflocculated and theremaining particles of W are sedimented.

The specific gravity of W is 19.24 (20° C.). Particles having a largespecific gravity (large-specific-gravity particles) which may beemployed instead of W are preferably metal particles of Mo (molybdenum),Ni (nickel), Ta (tantalum), Cr (chromium), or their alloys, the metalparticles having a high melting point, a specific gravity which is 1.5times that of the small-specific-gravity particles or greater, andparticle diameters distributed across the deflocculation limit.

Since NiCr is used as a sintering assistant for sintering W at lowtemperature, it is necessary for the sintering assistant of NiCr to bedeflocculated and sedimented in a manner similar to W. Therefore,particles of NiCr having a particle diameter ranging from about 3.5 μmto 5.2 μm are selected as shown in FIG. 13(c).

The sintering assistant which may be added instead of NiCr may compriseNi powder, Cr powder, Co powder, Cu powder, Ti powder, or theirmixtures, as shown in FIG. 12(b) (the steps following the mixing step inFIG. 12(b) are omitted from illustration as they are the same as thoseshown in FIG. 12(a)). The sintering assistant should preferably be addedin 5 to 50 parts with respect to 100 parts (by volume) of thelarge-specific-gravity particles such as of tungsten, molybdenum, or thelike. The sintering temperature varies depending on the selectedsintering assistant. For example, when the sintering assistant is ofNiCr and Ni, the sintering temperature ranges from 1300 to 1400° C., andwhen the sintering assistant is of Cu, the sintering temperature rangesfrom 1270 to 1320° C.

The above starting materials are weighed, and mixed with pure water, adeflocculant, and a binder in a monopot, after which an antifoamer isadded to the mixture. The mixture is then deaerated in vacuum and sievedby a sieve of 300 mesh to remove floating substances and contaminants.An antifoamer is further added to the mixture, and the mixture isdeaerated in vacuum, thus producing a mixed slurry S (ceramic/metalmixed slurry) containing a group of small-specific-gravity particles ofAl₂ O₃, etc. and a group of large-specific-gravity particles of W, etc.

The mixed slurry S is then poured into a plaster mold, deposited, anddried into a forming body, which is thereafter sintered in a reducingatmosphere thereby producing a gradient function material.

Instead of the plaster mold, there may be employed a porous resin moldmade of alumina cement, phenolic resin, or the like, or a porous ceramicmold. These molds should have pore diameters which are 2 or 3 timesthose of particles to be molded (equal to or smaller than those ofsecondary agglomerated particles). The material of the resin or ceramicmold should be selected which does not contain light elements (Mg or thelike) that adversely affects the sintered body.

Various steps to be carried out after a mixed slurry S is poured into aplaster mold until a gradient function material is produced will bedescribed below with reference to FIGS. 14(a) through 14(e). FIGS. 14(a)through 14(e) are cross-sectional views showing a process ofmanufacturing a sealing cap for a metal vapor discharge lamp as agradient function material.

As shown in FIG. 14(a), a container coated with wax on its innersurface, such as a glass tube 2, is set on a plaster mold 10 (such as amold which meets the requirements under JIS R9111), and the mixed slurryS is poured into the glass tube 2.

The region of the poured mixed slurry S which is held in contact withthe plaster mold 10 is deposited as dominated by being attracted by theplaster mold 10. As a result, the composition of the deposited region isthe same as the composition of the slurry S itself.

After the deposition dominated by being attracted by the plaster mold 10is completed, the mixed slurry S is deposited as dominated by thespecific gravity and the deflocculated condition. Specifically, all theparticles of Al₂ O₃ are difficult to be deposited as they aredeflocculated and suspended while being repelled by each other. Some ofthe particles of W are deflocculated, and the remaining particles of Ware not in a deflocculated condition. Therefore, after particles of Ware deposited as being dominated by being attracted by the plaster mold10, particles of W which are not in a deflocculated condition aresedimented.

The particles in the deflocculated condition are gradually deposited dueto gravity. At this time, the particle size distribution of theparticles of W is greater than the particle size distribution of theparticles of Al₂ O₃ and the specific gravity of W is greater than (morethan twice) the specific gravity of Al₂ O₃, as shown in FIGS. 13(a) and13(b). Therefore, as shown in FIG. 14(b) , there is produced a gradientfunction material forming body 3 with the composition of its gradientlayer varying continuously such that there exists alarge-specific-gravity region 3a which is occupied mostly bylarge-specific-gravity particles of W intermediate (in a position closeto the bottom in the second embodiment) in the transverse direction(casting direction), and the proportion of small-specific-gravityparticles of Al₂ O₃ progressively increases transversely from thelarge-specific-gravity region 3a.

As shown in FIG. 18, which is a graph showing the composition in thetransverse direction of the gradient function material forming body 3,the gradient function material forming body 3 exhibits a peakcomposition of a gradient function material in thelarge-specific-gravity region 3a. Since an end 3b of the gradientfunction material forming body 3 which is held in contact with theplaster mold 10 is formed as dominated by being attracted by the plastermold 1, the composition of the end 3b is close to the composition of themixed slurry S. The composition of an opposite end 3c is occupied mostlyby particles of Al₂ O₃.

Results of experimentation for properties (electric resistance, Vickershardness, and fracture toughness) along the deposited thickness of thegradient function material forming body 3 are shown in FIGS. 21, 22, and23. These experimental results indicate that the gradient functionmaterial forming body 3 has property peaks corresponding to the peakcomposition in the intermediate region 3a.

As described above with reference to FIG. 25, the gradient pattern of agradient function material manufactured by the conventional method issuch that the stripe laminae of the gradient layer are liable to peeloff because they are superposed too orderly in the transverse direction.As with the first embodiment, the gradient function material accordingto the second embodiment is not produced by deflocculating particlegroups in the slurry and sedimenting and depositing the particles asdominated by only the specific gravity. Therefore, the order of thegradient layer (which is dominated by the specific gravity) isdisturbed, resulting in a gradient pattern which continuously varies ina transverse direction thereof as if as if in transversely superposedcorrugated patterns, as shown in FIG. 15.

Since the gradient function material forming body 3 is used as a cap fora metal vapor discharge lamp in this embodiment, the portion below thelarge-specific-gravity region 3a is removed as by grinding after beingdried as shown in FIG. 14(c), leaving the gradient function materialforming body 3 free of peaks in the varying composition.

As shown in FIG. 14(d), the remaining gradient function material formingbody 3 (including regions 3a, 3c) is either temporarily fired for 1 houror directly processed to shape without being temporarily fired. When theremaining gradient function material forming body 3 is directlyprocessed to shape, it is shaped to a dimension that matches thediameter of the mouth of a light-emitting tube taking into account theshrinkage which it suffers upon firing.

Then, the gradient function material forming body 3 is fired at 1350° C.for 6 hours, after which it is processed by HIP in an argon atmosphere.The gradient function material forming body 3 is formed into acylindrical shape so as to be suitable for use as a sealing cap.Electrode holes 4a, 4b are defined in the gradient function materialforming body 3. Then, as shown in FIG. 14(e), an inner electrode 5a ispressed into the electrode hole 4a, and an outer electrode 5b is pressedinto the electrode hole 4b, thus producing a sealing cap 6. A metalsolder material may be put between the electrodes and the electrodeholes when the electrodes are pressed into the electrode holes.

FIG. 16(a) shows a bulb 100 for a metal vapor discharge lamp with thesealing cap 6 mounted thereon. The sealing cap 6 has a projection 6a onits distal end which is securely fitted in a recess 11a defined in anend of a tubular light-transmissive light-emitting tube 11 which is madeof polycrystalline alumina. For securing the sealing cap 6 to thetubular light-transmissive light-emitting tube 11, the portion of thesealing cap 6 to be held against an open end of the light-emitting tube11 is composed of the end 3c which is occupied mostly by particles ofAl₂ O₃, and the coefficient of thermal expansion of that portion of thesealing cap 6 is selected to be substantially equal to the coefficientof thermal expansion of light-transmissive alumina of which thelight-emitting tube 11 is made. The sealing cap 6 is secured to thetubular light-transmissive light-emitting tube 11 by placing a sealingmaterial 12 such as a glass solder or the like between the recess 11aand the projection 6a, and heating the sealing material 12 withhigh-frequency energy or infrared radiation.

FIG. 16(b) shows another type of sealing cap 6'. In this type, thesealing cap 6! has an outside diameter smaller than the outside diameterof the light-emitting tube 11. The other structural details are the sameas those shown in FIG. 16(a).

FIGS. 17(a) and 17(b) are views showing a method of manufacturing agradient function material according to a first modification of themethod of manufacturing a gradient function material according to thesecond embodiment shown in FIGS. 14(a) through 14(e). In themanufacturing method according to the first modification, as shown inFIG. 17(a), a single slurry S' containing only particles of Al₂ O₃(particles having a small specific gravity) is supplied into a glasstube 2 on a plaster mold 1 and deposited therein as a deposited layer3d. Then, as shown in FIG. 17(b), a mixed slurry S of particles of Al₂O₃ and particles of W is supplied into the glass tube 2, and theparticles in the mixed slurry S are deposited successively from theparticles of W that are more susceptible to gravity, onto the depositedlayer 3d of Al₂ O₃.

Now, as shown in FIG. 17(b), a gradient composition region whosecomposition continuously varies transversely is formed on the depositedlayer 3d which is composed of 100% of Al₂ O₃ with a clear boundarydefined therebetween. The gradient composition region is formed asdominated by the specific gravity and deflocculation, but notessentially affected by any attraction from the plaster mold 10, andhence is free of any peak compositions.

Thereafter, the deposited layer 3d is removed, producing a gradientfunction material forming body 3 for use as a material of the sealingcap 6. A mixed slurry S may be supplied twice, rather than supplying thesingle slurry S'. However, since the boundary would be blurred, it ispreferable to employ the single slurry S'. FIG. 19 is a graph showing acomposition in the transverse direction of a gradient function materialforming body shown in FIG. 17(b).

FIGS. 20(a) and 20(b) are views showing a method of manufacturing agradient function material according to a second modification of themethod of manufacturing a gradient function material according to thesecond embodiment shown in FIGS. 14(a) through 14(e).

In the manufacturing method according to the second modification, asshown in FIG. 20(a), a piston 7 is fitted in a bottom of a glass tube 2as a container, the piston 7 being slidable along an inner surface ofthe glass tube 2. Then, a mixed slurry S containing a group of particleshaving a small specific gravity and a group of particles having a largespecific gravity is supplied into the glass tube 2.

Thereafter, the mixed slurry S is held at rest for a predeterminedperiod of time until large-specific-gravity particles are of aprogressively greater proportion from upper toward lower layers in thecomposition of the mixed slurry S in the glass tube 2, i.e., the upperlayer is occupied mostly by particles of Al₂ O₃, whereupon a plastermold 10 is brought into contact with the upper surface of the mixedslurry S, and the piston 7 is elevated to pressurize the mixed slurry Sto deposit the slurry on the surface of the plaster mold 10, as shown inFIG. 20(b).

In this manner, there is produced a gradient function material whosecomposition varies only in one direction, without forming andsubsequently removing the deposited layer 3d as with the firstmodification.

As described above, the present invention offers the followingadvantages:

The gradient function material according to the present invention canachieve a required thickness in a much larger and complete range thanthe gradient function materials according to the conventional methods.Specifically, the gradient layer produced by vacuum evaporation such asCVD has a maximum thickness of several hundreds μm, and the gradientlayer produced by centrifugal separation or solid-liquid separationusing a filter has a minimum thickness of several mm, whereby there is alarge gap or empty region in the thickness for the gradient layer whichmay be achieved using conventional methods. With the method ofmanufacturing a gradient function material according to the presentinvention, it is possible to manufacture a gradient layer having athickness ranging from several hundreds μm to ten mm or more.

The composition of particles making up the gradient function materialcontinuously varies in a transverse direction of the gradient layerthereof as if in transversely superposed corrugated patterns. Therefore,the gradient function material is less liable to peel off in thin layersand is more resistant to thermal stresses than the conventional gradientfunction materials which are composed of laminae arranged regularly asstripes.

Since a gradient layer can be formed by deposition using a plaster moldand one type of slurry, any conventional dehydrating step becomesunnecessary, resulting in a simplified process.

Because no vacuum pump and no rotating device for producing centrifugalforces, or no filter is needed, the apparatus may be of a simplifieddesign, which gives an advantage as to the cost.

According to the second embodiment of the present invention, it ispossible to form a gradient function material having a composition peakintermediate in the transverse direction. If a gradient functionmaterial with no composition peak is required, then the above gradientfunction material may be processed to shape as an intermediate product.If a single slurry containing a group of particles having a smallspecific gravity is first supplied into a plaster mold and depositedtherein, and then a mixed slurry is supplied into the plaster mold, thensince the boundary between a deposited region (with no composition peak)formed from the mixed slurry and a deposited region formed from thesingle slurry can clearly be distinguished, the deposited region formedfrom the single slurry can easily be removed subsequently by grinding orthe like.

INDUSTRIAL APPLICABILITY

The gradient function material can be used as a heat resistant materialin locations where the difference between inner and outer temperaturesis very large, e.g., as a sealing cap for the bulb of a metal vapordischarge lamp, a surface layer material for a space shuttle, a nuclearfusion reactor, etc.

Although there have been described what are at present considered to bethe preferred embodiments of the invention, it will be understood thatthe invention can be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are, therefore, to be considered in all aspects asillustrative and not restrictive. The scope of the invention isindicated by the appended claims rather than by the foregoingdescription.

We claim:
 1. A gradient function material seal cap for a discharge lampbulb, the seal cap produced by molding and thereafter firing a slurrywhich contains a plurality of groups of particles having differentspecific gravities:said plurality of groups of particles include atleast a first group of particles and a second group of particles; saidfirst group of particles comprises a group of nonmetal particles havinga maximum particle diameter equal to or smaller than a deflocculationlimit of about 6.0 μm, said nonmetal particles being made of one or morematerials selected from the group consisting of alumina, zirconia,magnesia, silica, silicon carbide, titanium carbide, silicon nitride,and AlON; and said second group of particles comprises a group of metalparticles of a high melting point which have a specific gravity which isabout 1.5 times the specific gravity of said first group of particles,and particle diameters distributed across said deflocculation limit, andsaid metal particles of a high melting point being made of an alloycontaining at least one material selected from the group consisting ofnickel, tungsten, molybdenum, tantalum, and chromium.
 2. A gradientfunction material seal cap according to claim 1, wherein a sinteringassistant is added to said second group of particles for sintering thesecond group of particles at a low temperature.
 3. A gradient functionmaterial seal cap for a discharge lamp bulb, the seal cap produced bymolding and thereafter firing a slurry which contains a plurality ofgroups of particles having different specific gravities;said pluralityof groups of particles includes at least a first group of particles anda second group of particles; said first group of particles comprises agroup of nonmetal particles having a maximum particle diameter ≦ adeflocculation limit of about 6.0 μm, said nonmetal particles being madeof at least one material selected from the group consisting of alumina,zirconia, magnesia, silica, silicon carbide, titanium carbide, siliconnitride, and AlON; said second group of particles comprises a group ofmetal particles of a high melting point which have a specific gravitywhich is about 1.5 times the specific gravity of said first group ofparticles, and particle diameters distributed across the deflocculationlimit, and said metal particles of a high melting point being made of analloy containing at least one material selected from the groupconsisting of nickel, tungsten, molybdenum, tantalum, and chromium; asintering assistant being added to said second group of particles forsintering the second group of particles at a low temperature; and saidsintering assistant is added to said second group of particles in about5 to 50 parts with respect to 100 parts (by volume) of said second groupof particles.
 4. A gradient function material seal cap according toclaim 2, wherein said sintering assistant comprises powder containing atleast one material selected from the group consisting of NiCr powder, Nipowder, Cr powder, Co powder, Cu powder, and Ti powder.
 5. A gradientfunction material seal cap for a discharge lamp bulb, the seal capproduced by molding and thereafter firing a slurry which contains aplurality of groups of particles having different specificgravities;said plurality of groups of particles includes at least afirst group of particles and a second group of particles; said firstgroup of particles comprises a group of nonmetal particles having amaximum particle diameter ≦ a deflocculation limit of about 6.0 μm, saidnonmetal particles being made of at least one material selected from thegroup consisting of alumina, zirconia, magnesia, silica, siliconcarbide, titanium carbide, silicon nitride, and AlON; said second groupof particles comprises a group of metal particles of a high meltingpoint which have a specific gravity which is about 1.5 times thespecific gravity of said first group of particles, and particlediameters distributed across the deflocculation limit, and said metalparticles of a high melting point being made of an alloy containing atleast one material selected from the group consisting of nickel,tungsten, molybdenum tantalum, and chromium; and said groups ofparticles have a composition continuously varying in a transversedirection of said seal cap as if in transversely superposed corrugatedpatterns.
 6. A gradient function material seal cap according to claim 1,wherein said first group of particles have a specific gravity rangingfrom about 3 to
 7. 7. A gradient function material seal cap according toclaim 1, wherein said slurry is introduced into a porous mold to form amolded body which is then fired to form the seal cap.
 8. A gradientfunction material seal cap according to claim 1, wherein said seal capcomprises an electrically conductive layer located away from a dischargespace which is defined by the discharge lamp bulb sealed with the sealcap, and an inner electrode located within the discharge space andcoupled to the conductive layer without being extruded externally of theseal cap.
 9. A gradient function material seal cap according to claim 5,wherein said seal cap comprises an electrically conductive layer locatedaway from a discharge space which is defined by the discharge lamp bulbsealed with the seal cap, and an inner electrode located within thedischarge space and coupled to the conductive layer without beingextruded externally of the seal cap.
 10. A gradient function materialseal cap according to claim 2, wherein said low temperature is in arange of 1270-1400° C.